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What is happening in the uterus that leads to menstruation? OR

Why menstruation does not occur during pregnancy?


The basic phenomenon behind the uterine cycle is the flow of blood from vagina, originated from the uterine endometrium. The blood flow, i.e. bleeding is known as menstruation or menstrual period or menses.

This occurs due to the various changes in the female sex hormones like estrogen and progesterone that affects the wall of endometrium.

The whole process of changes take place in the cyclic form divided into 28 day cycle.

The uterine cycle is also known as menstrual cycle or endometrial cycle. Its phases are as follows:

I. Proliferative phase:

  • This phase is also known as preovulatory phase or postmenstrual phase or follicular phase.
  • It begins on the 6th day after the starting of menstruation and ends on the 14th day of the cycle.
  • During this phase the glands are lengthen and become more vascular whereas the arteries become coiled.
  • This shows some proliferative changes during the cycle.

II. Secretory phase:

  • This phase is also known as postovulatory or premenstrual or luteal or progestational phase.
  • It begins on the 15th day and ends on the 27th day of the cycle.
  • At the end of the cycle, endometrium becomes thick about 5-6mm and shows marked hypertrophy.
  • During these days the glands become elongated, coiled and distended with mucus.
  • There is an increase in the glandular secretion, due to that increase in the number of spiral arteries and blood flow is observed.
  • The secretion of glands is mucoid in nature and the vessels are congested.
  • This whole environment is very suitable for the fertilization but if it doesn’t occur, it bleeds.

III. Destructive phase:

  • This phase is known as the phase of menstrual blood flow or the phase of bleeding or menstruation.
  • This phase is the end of the menstrual cycle, in which the blood flow (circulation) becomes slow down in the uterus.
  • This leads to the vasoconstriction of the spiral arteries that cause ischaemia that leads to the destruction of endometrium called as necrosis.
  • The bits of necrosed endometrium are shed, the vessels then dilate and bleeding occurs.

Day 1 to 5: due to lower concentration of female hormones, the endometrium disintegrates and the blood vessels break up. The secretion of blood ooze out of the vaginal tract.

Day 6 to 13: concentration of estrogen hormone increases, this leads to the formation of glandular, vascularized and thick endometrium.

Day 14: ovulation occurs

Day 15 to 28: the concentration of progesterone increases, due to which corpus luteum forms, development of endometrium occurs and also become ready for the implantation of embryo via fertilization. If implantation doesn’t occur then disintegration of corpus luteum takes place. This promotes the inhibition of progesterone secretion and due to that endometrium disintegrates. During this period secretion of FSH (follicle stimulating hormone) begins and formation of new ovarian follicle takes place.

If fertilization occurs, the endometrium changes progress towards the pregnancy and menstruation cycle disappears.

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